With rapid industrialization and industrial development, consumption of resources such as petroleum has increased, and stable production and supply of oil have become a crucial global problem.
For this reason, development of smaller marginal fields or deep-sea oil fields, which has been ignored due to lack of economic feasibility, is now emerging. Thus, development of offshore plants provided with oil drilling equipment suitable for use in such oil fields is actively carried out along with development of seabed mining technology.
An offshore plant is provided with various types of drilling-related equipment such as a Derrick crane system, a draw-works, a top drive, a mud pump, a cement pump, a riser, and a drill pipe.
The draw-works performs lifting of the drill pipe, insertion of a casing, and the like, and includes a drum and a motor. The drum is powered by the motor so as to wind or unwind a wire rope for controlling lifting of the drill pipe. The rotational speed of the motor can be adjusted so as to adjust the rotational speed of the drum, thereby enabling adjustment of the speed of the drill pipe.
The top drive provides power for drilling and pipe fastening in drilling operation.
Offshore plants are divided into a stationary platform anchored at one point of nearshore to perform drilling operations and a floating offshore plant capable of performing drilling operation at ocean depths of 3,000 m or more.
The floating offshore plant is provided with a plurality of thrusters as a main propulsion device or a propulsion device for computer-aided dynamic positioning. The thrusters are located at the bottom of a ship to change an operating direction of a propeller and are commonly used to allow the ship to navigate or to sail in a canal or enter/leave a port under its own power without a tug. The thruster is powered by a thrust motor connected thereto.
FIG. 1 is a diagram of a typical power supply system known in the art.
Referring to FIG. 1, AC power generated by a power generator 110 is supplied to an AC bus, to which a first AC/DC converter 121, a second AC/DC converter 122, and a third AC/DC converter 123 are connected.
The first AC/DC converter 121 converts alternating current supplied from the AC bus into direct current and supplies the direct current to a first DC bus 131, and a DC/AC converter 141 converts direct current supplied from the first DC bus 131 into alternating current and supplies the alternating current to a first thruster motor 151.
The second AC/DC converter 122 converts alternating current supplied from the AC bus into direct current and supplies the direct current to a second DC bus 132, and a DC/AC converter 142 converts direct current supplied from the second DC bus 132 into alternating current and supplies the alternating current to a second thruster motor 152.
The third AC/DC converter 123 converts alternating current supplied from the AC bus into direct current and supplies the direct current to a third DC bus 133, and a plurality of DC/AC converters 143 to 148 are connected to the third DC bus 133. The plurality of DC/AC converters 143 to 148 convert direct current supplied from the third DC bus 133 into alternating current and supply the alternating current to corresponding ones of a plurality of draw-works motors 153, 154, 155, 158, 159 and a plurality of top drive motors 156, 157.
Since the draw-works motors 153, 154, 155, 158, 159 and the top drive motors 156, 157 are configured to repeatedly lift or lower drilling equipment such as a drill pipe, a brake is frequently put on the motors to bring the motors to a sudden stop or to rotate the motors in a reverse direction during rotation at rated load, and, for the thruster motors 151, 152, a brake is also frequently put on the motors to bring the motors to a sudden stop or to rotate the motors in the reverse direction during rotation at rated load. When a brake is put on the motors, regenerative power is generated in the motors. In addition, when a thruster is rotated due to a disturbance, regenerative power is also generated in the thruster motors.
When regenerative power is generated in the draw-works motors, the top drive motors, or the thruster motors, voltage of a DC bus connected to the draw-works motors, the top drive motors, or the thruster motors is increased, and, when the voltage is increased beyond a degree which the DC bus can accommodate, the DC bus is tripped.
Thus, the typical power supply system is provided with resistors 161 to 166 to consume regenerative power as heat, thereby preventing the DC bus from tripping.
FIG. 2 is a graph showing power consumption of each component of a typical power supply system known in the art.
Referring to FIG. 2, electric power produced by a power generator is supplied to a first load 220 and an AC/DC converter 260 through a distributor. The AC/DC converter 260 converts alternating current into direct current and supplies the direct current to a second load 240 while supplying the direct current to a draw-works 230 through a DC/AC converter. Each of the first load 220 and the second load 240 is a load that consumes a constant level of electric power. On the contrary, power consumption of the draw-works 230 changes continuously, and, in FIG. 2, a negative value of electric power means that regenerative power is generated. Regenerative power generated in the draw-works 230 is consumed by the second load or a resistor 250.
In FIG. 2, it can be seen that abrupt change in power consumption of the draw-works 230 causes abrupt change in power output of the power generator 210. A diesel power generator consumes more fuel and discharges more exhaust gas in the case where power output is maintained at a constant level than in the case where power output is abruptly changed.
In order to supply suitable electric power corresponding to abrupt change in power consumption of the draw-works motors, the top drive motors, and the thruster motors, it is necessary to rapidly change power output of the power generator. However, the power generator has difficulty providing suitable electric power corresponding to abrupt change in power consumption of the draw-works motors, the top drive motors, and the thruster motors due to low responsiveness thereof. If suitable power supply to the draw-works motors, the top drive motors, and the thruster motors is not achieved, a dangerous situation can occur due to characteristics of drilling operation. Further, when power supply to the draw-works motors or the top drive motors is suddenly interrupted upon loss of power, a dangerous situation can also occur.
In other words, typical power supply systems have problems such as energy waste due to consumption of regenerative power by resistors, increase in fuel consumption and exhaust gas due to abrupt change in power output of a power generator, difficulty in suitable electric power supply to draw-works motors, top drive motors, and thruster motors, and danger upon loss of power.